A free of cost TLD is probably not practical. There are some special things that go along with being a TLD and being in the public suffix list: https://github.com/publicsuffix/list

If you drop the TLD part though, you can do whatever you want with any domain you want, up to and including handing out free subdomains to anyone who asks. As usual, though, if you try to do this, the dark internet will make you regret it as one day, quite suddenly, you'd find you were hosting the DNS of some massive scam email or other, or any of who-can-even-enumerate ways of making you sorry you put this service up because of their abuse of it. Just like anyone can make a URL redirector, and many people even use it as a sort of "learn this language project" but if you actually put it up online you will rapidly regret it.

It's a bummer and I'm not celebrating this fact, but, yeah, it's not something you want to do.